Biography

News

Research

Research interests

Research interests include turbulent flow and dispersion, with applications to environmental aerodynamics and air pollution problems. Work includes experimental and mathematical modelling of turbulent flows, mixing processes and concentration fluctuations, the dispersion of emissions in the atmosphere and their subsequent deposition, wind tunnel methods for simulating atmospheric flow and dispersion, and wind power studies.

This paper investigates an odour incident that occurred in April 2008 with an initially unknown source and cause which resulted in hundreds of notifications of odour complaints across England. Detailed analysis of the incident illustrates how a combination of the geographical distribution of odour reports together with Met Office data and back-trajectory modelling can be utilised to trace the source location and source term of the odour. The analysis suggests that the source of the odorant was not locally generated and that long range transport from Northern Europe was the likely explanation. This requires potentially exceptional source strength so that dilution at distance is sufficient to lead to odour perception thousands of km away. The proposed cause is suggested to be wide-spread spreading of agricultural slurry. This is common practice in Europe during the spring, and has implications for future reports of odour travelling extensive distances and resulting in long-range pollution events.

This paper discusses waste management in the UK and its relationship with health. It aims to outline the role of health professionals in the promotion of waste management, and argues for a change in their role in waste management regulation to help make the process more sustainable. The most common definition of sustainable development is that by the Brundtland commission, i.e. "development that meets the needs of the present without compromising the ability of future generations to meet their own needs". Managing waste sites in a manner that minimises toxic impacts on the current and future generations is obviously a crucial part of this. Although the management of waste facilities is extremely complex, the Integrated Pollution Prevention and Control regime, which requires the input of public health professionals on the regulation of such sites, means that all waste management installations should now be operating in a fashion that minimises any toxicological risks to human health. However, the impacts upon climate change, resource use and health inequalities, as well as the effects of waste transportation, are currently not considered to be part of public health professionals' responsibilities when dealing with these sites. There is also no requirement for public health professionals to become involved in waste management planning issues. The fact that public health professionals are not involved in any of these issues makes it unlikely that the potential impacts upon health are being considered fully, and even more unlikely that waste management will become more sustainable. This paper aims to show that by only considering direct toxicological impacts, public health professionals are not fully addressing all the health issues and are not contributing towards sustainability. There is a need for a change in the way that health professionals deal with waste management issues.

This work presents selected results of an EPSRC-funded project investigating the dispersion of nanoparticles in the wake of moving vehicles. The aims were to study the changes in particle number distribution (PND) due to the competing effects of dilution and transformation processes (e.g. coagulation, nucleation, condensation) over the travel time from tailpipe to roadside, and to model the fate of these particles in the near and the far wake regions of a moving vehicle. To achieve these objectives, firstly ground-fixed and on-board measurements of PNDs were performed using a fast response (sampling frequency up to 10Hz) differential mobility spectrometer (Cambustion DMS50) in the wake of a diesel engined car moving at a range of speeds from 20 to 50 km h-1. Secondly, wind tunnel simulations were carried out on reduced scale (1:5 and 1:20) models of the car used for the field experiments. The flow and turbulence fields were characterised both in the near and far wake of the modelled vehicle by using a two component laser Doppler anemometer. Concentration measurements were obtained by using a fast response (frequency >350 Hz) flame ionisation detector and a hydrocarbon tracer gas released from the modelled tailpipe. A high resolution experimental data base was obtained from both the field and wind tunnel measurements for formulating the basis of fast mathematical parameterisations that can be used with operational nanoparticle dispersion models.

The study of ultrafine particles (those below 100 nm in diameter) is of great interest to the scientific community and policy makers due to their likely impacts on human health and the environment. Understanding the behaviour of ultrafine particles from their number concentrations and size distribution point of view in the ambient air will help to expedite the development of regulatory controls. Vegetation barriers are used in many places to reduce the pollution generated by the road traffic from reaching to the people living in urban areas, especially close to the road, where the ultrafine particles are expected to be in high concentrations. Limited information currently exist that could reveal detailed understanding about the effectiveness of near road vegetation barriers in removing concentrations of ultrafine particles. A fast response differential mobility spectrometer (DMS50) is used for the pseudo-simultaneous measurements of number and size distributions in the 5-560 nm size range. The measurements were made at four different points along the side of a busy highway. These points were at the front, middle and back of the vegetation barrier, and at a point without any vegetation; all these points were at the same height above the road level. The data was collected at 10 Hz sampling rate, with T10-90% equal to 500 milliseconds, during a weekday (7 August 2012) and a weekend (11 August 2012). Analysis of the data was performed to investigate the influence of near road vegetative barriers on the number concentration and size distributions. Further analysis will be carried out to develop understanding about the effect of wind direction on the efficiency of the vegetation barrier and an indication about the dispersion of particles as they move away from source (vehicle tailpipe) through the vegetation barriers to roadside footpath. Preliminary results based on the weekday data shows that the concentrations of particles gradually decrease while passing through the vegetation barrier. No clear trend was found from the weekend data due to winds being parallel to road and low traffic density. Detailed analysis of the data is currently underway.

Kumar P, Fennell P, Robins A(2010)Comparison of the behaviour of manufactured and other airborne nanoparticles and the consequences for prioritising research and regulation activities, JOURNAL OF NANOPARTICLE RESEARCH12(5)pp. 1523-1530 SPRINGER

Understanding the transformation of nanoparticles emitted from vehicles is essential for developing appropriate methods for treating fine scale particle dynamics in dispersion models. This article provides an overview of significant research work relevant to modelling the dispersion of pollutants, especially nanoparticles, in the wake of vehicles. Literature on vehicle wakes and nanoparticle dispersion is reviewed, taking into account field measurements, wind tunnel experiments and mathematical approaches.
Field measurements and modelling studies highlighted the very short time scales associated with nanoparticle transformations in the first stages after the emission. These transformations strongly interact with the flow and turbulence fields immediately behind the vehicle, hence the need of characterising in detail the mixing processes in the vehicle wake. Very few studies have analysed this interaction and more research is needed to build a basis for model development. A possible approach is proposed and areas of further investigation identified.

The aim of this study is to assess particle number concentrations (PNCs) and distributions (PNDs) in a car cabin while driving. Further objectives include the determination of the influence of particle transformation processes on PNCs, PNDs and estimation of PNC related exposure. On-board measurements of PNCs and PNDs were made in the 5?560 nm size range using a fast response differential mobility spectrometer (DMS50), which has a response time of 500 ms. Video records of the traffic ahead of the experimental car were also used to correlate emission events with measured PNCs and PNDs. A total of 30 return trips was made on a 2.7 km route during morning and evening rush hours, with journey times of 7 ± 2 and 10 ± 3 min, respectively. The average PNC for the set of morning journeys, 5.79 ± 3.52 × 104 cm?3, was found to be nearly identical to the average recorded during the afternoon, 5.95 ± 4.67 × 104 cm?3. Average PNCs for individual trips varied from 2.42 × 104 cm?3 to 2.18 × 105 cm?3, mainly due to changes in the emissions affecting the experimental car (e.g. when the experimental car was following another vehicle). The largest one second averaged PNC during a specific event, 1.85 × 106 cm?3, was found to be over 30-times greater than the overall average of 5.87 ± 4.06 × 104 cm?3. Correlation of video records and concentration data indicated that close proximity to a preceding vehicle led to a clear increase in PNCs of freshly emitted nucleation mode particles. The evolution of normalised PNDs demonstrated that dilution was the dominant transformation process in the car cabin. The deposition of inhaled particles in the lung was estimated on the basis of either the size-resolved distribution or the total PNC. In general, the two methods yielded similar results but differences up to 30% were noted in some cases, with the latter method giving the lower values. Overall, the results reflect the importance of size-resolved measurements for deriving accurate evaluations of exposure rates, as well as identifying emissions from nearby traffic as the cause of short-term elevations of PNCs and hence dose rates.

In the present paper we have analysed experimentally (wind tunnel) and numerically (CFD) the impact of some morphological parameters on the flow within and above the urban canopy. In particular, this study is a first attempt in systematically studying the flow in and above urban canopies using simplified, yet more realistic than a simple array of cuboids, building arrays. Current mathematical models would provide the same results for the six case studies presented here (two models by three wind directions), however the measured spatially averaged profiles are quite different from each other.

Results presented here highlight that the differences in the spatially averaged vertical profiles are actually significant in all six experimental/numerical cases. Besides the building height variability, other morphological features proved to be a significant factor in shaping flow and dispersion at the local to neighbourhood scale in the urban canopy and directly above: building aspect ratio (or, conversely, the street canyon aspect ratio), the angle between the street canyons and the incoming wind and local geometrical features such as, for example, the presence of much taller buildings immediately upwind of the studied area.

Methods used to convert wind tunnel and ADMS concentration feld data for a
complex building array into effective radiation dose were developed based on
simulations of a site in central London. Pollutant source terms were from positron
emitting gases released from a cyclotron and clinical PET radiotracer facility.
Five years of meteorological data were analysed to determine the probability
distribution of wind direction and speed. A hemispherical plume cloud model
(both static and moving) was developed which enabled an expression of
gamma-ray dose, taking into account build-up factors in air, in terms of analytic
functions in this geometry. The standard building wake model is presented, but
this is extended and developed in a new model to cover the concentration feld
in the vicinity of a roof top structure recirculation zone, which is then related
to the concentration in the main building wake zone. For all models presented
the effective dose was determined from inhalation, positron cloud immersion
and gamma ray plume contributions. Results of applying these models for
determination of radiation dose for a particular site are presented elsewhere.

Wind tunnel experiments have been carried out at the EnFlo laboratory to measure mean and turbulent
tracer fluxes in geometries of real street canyon intersections. The work was part of the major DAPPLE
project, focussing on the area surrounding the intersection between Marylebone Road and Gloucester
Place in Central London, UK. Understanding flow and dispersion in urban streets is a very important issue
for air quality management and planning, and turbulent mass exchange processes are important
phenomena that are very often neglected in urban modelling studies. The adopted methodology involved
the combined use of laser Doppler anemometry and tracer concentration measurements. This methodology
was applied to quantify the mean and turbulent flow and dispersion fields within several street
canyon intersections. Vertical profiles of turbulent tracer flux were also measured. The technique, despite
a number of limitations, proved reliable and allowed tracer balance calculations to be undertaken in the
selected street canyon intersections. The experience gained in this work will enable much more precise
studies in the future as issues affecting the accuracy of the experimental technique have been identified
and resolved.

Commuters are regularly exposed to short-term peak concentration of traffic produced nanoparticles (i.e. particles <300 nm in size). Studies indicate that these exposures pose adverse health effects (i.e. cardiovascular). This study aims to obtain particle number concentrations (PNCs) and distributions (PNDs) inside and outside a car cabin whilst driving on a road in Guildford, a typical UK town. Other objectives are to: (i) investigate the influences of particle transformation processes on particle number and size distributions in the cabin, (ii) correlate PNCs inside the cabin to those measured outside, and (iii) predict PNCs in the cabin based on those outside the cabin using a semi-empirical model. A fast response differential mobility spectrometer (DMS50) was employed in conjunction with an automatic switching system to measure PNCs and PNDs in the 5?560 nm range at multiple locations inside and outside the cabin at 10 Hz sampling rate over 10 s sequential intervals. Two separate sets of measurements were made at: (i) four seats in the car cabin during

The statistics of the fluctuating concentration field within a plume is important in the analysis of atmospheric dispersion of toxic, inflammable and odorous gases. Previous work has tended to focus on concentration fluctuations in single plumes released in the surface layer or at ground level and there is a general lack of information about the mixing of two adjacent plumes and how the statistical properties of the concentration fluctuations are modified in these circumstances. In this work, data from wind tunnel experiments are used to analyse the variance, skewness, kurtosis, intermittency, probability density function and power spectrum of the concentration field during the mixing of two identical plumes and results are compared with those obtained for an equivalent single plume. The normalised variance, skewness and kurtosis on the centre-lines of the combined plume increase with distance downwind of the stack and, in the two-source configuration, takes lower values than those found in the single plumes. The results reflect the merging process at short range, which is least protracted for cases in which the sources are in-line or up to 30 {Mathematical expression} off-line. At angles of 45 {Mathematical expression} and more, the plumes are effectively side-by-side during the merging process and the interaction between the vortex pairs in each plume is strong. Vertical asymmetry is observed between the upper and the lower parts of the plumes, with the upper part having greater intermittency (i.e. the probability that no plume material is present) and a more pronounced tail to the concentration probability distribution. This asymmetry tends to diminish at greater distances from the source but occurs in both buoyant and neutral plumes and is believed to be associated with the 'bending-over' of the emission in the cross-flow and the vortex pair that this generates. The results allowed us to identify three phases in plume development. The first, very near the stack, is dominated by turbulence generated within the plume and characterised by concentration spectra with distinct peaks corresponding to scales comparable with those of the counter-rotating vortex pair. A second phase follows at somewhat greater distances downwind, in which there are significant contributions to the concentration fluctuations from both the turbulence internal to the plume and the external turbulence. The third phase is one in which the concentration fluctuations appear to be contr

A radiological assessment was carried out on the release of positron-emitting radioactive gases from a roof-level stack at a central London site. Different modelling approaches were performed to investigate the range of radiation doses to representative persons. Contributions from plume inhalation, gamma shine and immersion to effective dose were taken into account. Dry and wet surface deposition on the roof, and exposure from contamination on the skin of roof-workers, added only a mean 4.7% to effective dose and were neglected. A 1:200 scale model, consisting of the stack and surrounding buildings, was tested in a wind tunnel to simulate pollutant dispersion in the near-field region i.e. rooftop. Concentration field measurements in the wind tunnel were converted into effective dose, including for roof-workers installing glass cladding to the stack building. Changes in the building shape, from addition of the cladding layer, were investigated in terms of the near-field flow pattern and significant differences found between the two cases. Pollutant concentrations were also modelled using Air Dispersion Modelling System (ADMS) and the results used to calculate the effective dose using the same meteorological data set and source release terms. Sector averaged wind tunnel dose estimates were greater than the ADMS figure by approximately a factor of two to three. Different stack release heights were investigated in the wind tunnel and ADMS simulations in order to determine the best height for the replacement flue stack for the building. Other techniques were investigated: building wake models, modified Gaussian plume methods and uniform dilution into a hemispherical volume to show the wide variation in predicted dose possible with different approaches. Large differences found between simpler analytic approaches indicated that more robust radiological assessments, based on more complex modelling approaches, were required to achieve satisfactory estimates of radiation dose to representative groups in adjacent buildings and on the building rooftop.

Wind tunnel measurements downwind of reduced scale car models have been made to study the wake regions in detail, test the usefulness of existing vehicle wake models, and draw key information needed for dispersion modelling in vehicle wakes. The experiments simulated a car moving in still air. This is achieved by (i) the experimental characterisation of the flow, turbulence and concentration fields in both the near and far wake regions, (ii) the preliminary assessment of existing wake models using the experimental database, and (iii) the comparison of previous field measurements in the wake of a real diesel car with the wind tunnel measurements. The experiments highlighted very large gradients of velocities and concentrations existing, in particular, in the near-wake. Of course, the measured fields are strongly dependent on the geometry of the modelled vehicle and a generalisation for other vehicles may prove to be difficult. The methodology applied in the present study, although improvable, could constitute a first step towards the development of mathematical parameterisations. Experimental results were also compared with the estimates from two wake models. It was found that they can adequately describe the far-wake of a vehicle in terms of velocities, but a better characterisation in terms of turbulence and pollutant dispersion is needed. Parameterised models able to predict velocity and concentrations with fine enough details at the near-wake scale do not exist.

The CFD model Fluidyn-Panache was configured to model
atmospheric transport from an area source. Modelled flow and turbulence
were evaluated by comparison with on-site meteorological measurements,
whilst atmospheric dispersion was compared with wind tunnel measurements.
The results showed that higher rates of vertical and lateral dispersion were
modelled than were determined in the wind tunnel, though modelled and
measured ground-level centreline concentration data were within a factor
of two. Uncertainties in wind tunnel and numerical modelling were highest
close to the source. Consideration of fine-scale features was only necessary for
receptors in the immediate near-field.

Kumar P, Robins A(2010)Modelling the dispersion of nanoparticles in street canyons, HARMO 2010 - Proceedings of the 13th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposespp. 712-716

Suitable dispersion models are required for the prediction of nanoparticle number concentrations for adopting mitigation policies. The aim of this work is to model the dispersion of nanoparticle number concentrations in the 10-300 nm range at different heights in an urban street canyon. A modified Box model (Kumar et al. 2009b) and an operational street pollution model (OSPM) are used for this purpose and modelled results are compared with the measured nanoparticle concentrations. Further, the article discusses the role of particle dynamics in street-scale modelling and analyses the influence of the uncertainty in particle number emission factors on modelled concentrations. Reasons for discrepancies in modelled results due to particle number emission factors and street-level particle dynamics are given.

Contini D, Robins AG(2004)Experiments on the rise and mixing in neutral crossflow of plumes from two identical sources for different wind directions, Atmospheric Environment38(22)pp. 3573-3583 Elsevier

Following a malicious or accidental release in an outdoor environment (industrial or urban), first responders will ensure public safety by cordoning off and/or evacuating areas where human life may be in danger. Information on the source (strength and location) and the type of chemical agent released is needed for this to happen reasonably promptly and accurately.

A simple inverse modelling technique has been developed to estimate the source strength and location of such a release using measurements of concentration from chemical sensors. The technique relies on either a fixed installation or rapid deployment of chemical sensors to gather and return data to a base station. These measurements are there used, together with meteorological information, as the input data to an inverse algorithm that attempts to make a ?best? estimate of the source strength and location. The algorithm works to minimise a penalty function that measures the difference between the concentration observations and predictions based on the current estimate of the source parameters. This is an iterative procedure that should converge to a best estimate of those parameters and, in doing so, provide a measure of the uncertainty in that estimate. There is, in this, a trade-off between the desire for an early prediction and the error implicit in that prediction.

Wind tunnel experiments have been used to investigate the propagation of error through the inverse modelling procedure. Firstly, very detailed dispersion measurements were made in a deep boundary layer so that an accurate dispersion model could be established. Four fast flame ionisation detectors were then used to provide long, simultaneous concentration records in the plume from a ground level point source. The output was used to study the sensitivity to sensor placement and then sample duration. Simultaneous sub-samples were taken from the main records and used with the inversion algorithm to quantify the degradation of its performance with decreasing sample duration; i.e. with increasing uncertainty in the concentration observations. Guidelines for application of the inversion technique could then be proposed. The final stage was to move from a simple Gaussian plume to an urban dispersion model, in this case a street network model.

Scalar dispersion from ground-level sources in arrays of buildings is investigated using wind-tunnel measurements and large-eddy simulation (LES). An array of uniform-height buildings of equal dimensions and an array with an additional single tall building (wind tunnel) or a periodically repeated tall building (LES) are considered. The buildings in the array are aligned and form long streets. The sensitivity of the dispersion pattern to small changes in wind direction is demonstrated. Vertical scalar fluxes are decomposed into the advective and turbulent parts and the influences of wind direction and of the presence of the tall building on the scalar flux components are evaluated. In the uniform-height array turbulent scalar fluxes were dominant, whereas the tall building causes an increase of the magnitude of advective scalar fluxes which become the largest component. The presence of the tall building causes either an increase or a decrease to the total vertical scalar flux depending on the position of the source with respect to the tall building. The results of the simulations can be used to develop parametrizations for street canyon dispersion models and enhance their capabilities in areas with tall buildings.

This study compared dispersion calculations using a street network model (SIRANE) with results from wind tunnel experiments in order to examine model performance in simulating short-range pollutant dispersion in urban areas. The comparison was performed using a range of methodologies, from simple graphical comparisons (e.g. scatter plots) to more advanced statistical analyses. A preliminary analysis focussed on the sensitivity of the model to source position, receptor location, wind direction, plume spread parameterisation and site aerodynamic parameters. Sensitivity to wind direction was shown to be by far the most significant. A more systematic approach was then adopted, analysing the behaviour of the model in response to three elements: wind direction, source position and small changes in geometry. These are three very critical aspects of fine scale urban dispersion modelling. The overall model performance, measured using the Chang and Hanna (2004) criteria can be considered as ?good?. Detailed analysis of the results showed that ground level sources were better represented by the model than roof level sources. Performance was generally ?good? for wind directions that were very approximately diagonal to the street axes, while cases with wind directions almost parallel (within 20°) to street axes gave results with larger uncertainties (failing to meet the quality targets). The methodology used in this evaluation exercise, relying on systematic wind tunnel studies on a scaled model of a real neighbourhood, proved very useful for assessing strengths and weaknesses of the SIRANE model, complementing previous validation studies performed with either on-site measurements or wind tunnel measurements over idealised urban geometries.

We present results from laboratory and computational experiments on the turbulent flow over an array of rectangular blocks modelling a typical, asymmetric urban canopy at various orientations to the approach flow. The work forms part of a larger study on dispersion within such arrays (project DIPLOS) and concentrates on the nature of the mean flow and turbulence fields within the canopy region, recognis- ing that unless the flow field is adequately represented in computational models there is no reason to expect realistic simulations of the nature of the dispersion of pollutants emitted within the canopy. Comparisons between the experimental data and those ob- tained from both large-eddy simulation (LES) and direct numerical simulation (DNS) are shown and it is concluded that careful use of LES can produce generally excellent agreement with laboratory and DNS results, lending further confidence in the use of LES for such situations. Various crucial issues are discussed and advice offered to both experimentalists and those seeking to compute canopy flows with turbulence resolving models

A family of wall models is proposed that exhibits more satisfactory performance than
previous models for the large-eddy simulation (LES) of the turbulent boundary layer over a rough
surface. The time and horizontally averaged statistics such as mean vertical profiles of wind velocity,
Reynolds stress, turbulent intensities, turbulent kinetic energy and also spectra are compared with
wind-tunnel experimental data. The purpose of the present study is to obtain simulated turbulent
flows that are comparable with wind-tunnel measurements for use as the wind environment for the
numerical prediction by LES of source dispersion in the neutral atmospheric boundary layer.

Pollutant mass fluxes are rarely measured in the laboratory, especially their turbulent
component. They play a major role in the dispersion of gases in urban
areas and modern mathematical models often attempt some sort of parametrisation.
An experimental technique to measure mean and turbulent fluxes in an
idealised urban array was developed and applied to improve our understanding
of how the fluxes are distributed in a dense street canyon network. As expected,
horizontal advective scalar fluxes were found to be dominant compared with the
turbulent components. This is an important result because it reduces the complexity
in developing parametrisations for street network models. On the other
hand, vertical mean and turbulent fluxes appear to be approximately of the
same order of magnitude. Building height variability does not appear to affect
the exchange process significantly, while the presence of isolated taller buildings
upwind of the area of interest does. One of the most interesting results, again,
is the fact that even very simple and regular geometries lead to complex advective
patterns at intersections: parametrisations derived from measurements
in simpler geometries are unlikely to capture the full complexity of a real urban
area.

The need to balance computational speed and simulation accuracy is a key challenge in designing atmospheric dispersion models that can be used in scenarios where near
real-time hazard predictions are needed. This challenge is aggravated in cities, where models need to have some degree of building-awareness, alongside the ability to capture effects
of dominant urban flow processes. We use a combination of high-resolution large-eddy simulation (LES) and wind-tunnel data of flow and dispersion in an idealised, equal-height
urban canopy to highlight important dispersion processes and evaluate how these are reproduced by representatives of the most prevalent modelling approaches: (i) a Gaussian plume model, (ii) a Lagrangian stochastic model and (iii) street-network dispersion models. Concentration data from the LES, validated against the wind-tunnel data, were averaged over
the volumes of streets in order to provide a high-fidelity reference suitable for evaluating
the different models on the same footing. For the particular combination of forcing wind direction and source location studied here, the strongest deviations from the LES reference
were associated with mean over-predictions of concentrations by approximately a factor of
2 and with a relative scatter larger than a factor of 4 of the mean, corresponding to cases
where the mean plume centreline also deviated significantly from the LES. This was linked
to low accuracy of the underlying flow models/parameters that resulted in a misrepresentation of pollutant channelling along streets and of the uneven plume branching observed in
intersections. The agreement of model predictions with the LES (which explicitly resolves
the turbulent flow and dispersion processes) greatly improved by increasing the accuracy
of building-induced modifications of the driving flow field. When provided with a limited
set of representative velocity parameters, the comparatively simple street-network models
performed equally well or better compared to the Lagrangian model run on full 3D wind
fields. The study showed that street-network models capture the dominant building-induced
dispersion processes in the canopy layer through parametrisations of horizontal advection
and vertical exchange processes at scales of practical interest. At the same time, computational costs and computing times associated with the network approach are ideally suited
for emergency-response applications.

Research under the Managing Air for Green Inner Cities (MAGIC) project uses measurements and modelling to investigate the connections between external and internal conditions: the impact of urban airflow on the natural ventilation of a building. The test site was chosen so that under different environmental conditions the levels of external pollutants entering the building, from either a polluted road or a relatively clean courtyard, would be significantly different. Measurements included temperature, relative humidity, local wind and solar radiation, together with levels of carbon monoxide (CO) and carbon dioxide (CO2) both inside and outside the building to assess the indoor?outdoor exchange flows. Building ventilation took place through windows on two sides, allowing for single-sided and crosswind-driven ventilation, and also stack-driven ventilation in low wind conditions. The external flow around the test site was modelled in an urban boundary layer in a wind tunnel. The wind tunnel results were incorporated in a large-eddy-simulation model, Fluidity, and the results compared with monitoring data taken both within the building and from the surrounding area. In particular, the effects of street layout and associated street canyons, of roof geometry and the wakes of nearby tall buildings were examined.